Direct current oil-filled self contained cable

ABSTRACT

A DC OF cable has its main insulation composed of a composite insulation tape comprising a low dielectric loss film and kraft paper (PPLP). 1-10 sheets of kraft paper are wound on the PPLP main insulation as a layer on the inside and/or the outside thereof.

TECHNICAL FIELD

The present invention relates to a direct current (hereinafterabbreviated to DC) oil-filled self contained (hereinafter abbreviated toOF) cable used as a long power cable, particularly a long distancesubmarine power cable, for large electric power transmission.

PRIOR ART

Heretofore, oil immersed solid power cables, or mass-impregnatednon-draining (MIND) cables and OF cables have been adaptable as DCcables. In the case of DC OF cables, kraft paper with highimpermeability and which is excellent in breakdown strength is oftenused, because such dielectric characteristics as permittivity ordielectric tangent are, unlike alternating current (hereinafterabbreviated to AC) cables, not significant to DC cables. They areotherwise not much different from AC cables in basic structure.

DC Cables must have (1) sufficient DC withstanding characteristics, (2)sufficient withstanding characteristics against such abnormal voltagesas lightening surges etc. and (3) sufficient withstandingcharacteristics against polarity reversal and, moreover, must satisfythe required carrying capacity.

DC stress distribution in insulation is determined by theρ-characteristic of the insulation. This ρ-characteristic in turn varieswith temperature and stress. It undergoes a complex change, as thetemperature distribution in the insulation changes.

As far as the conventional DC cables in use with the kraft paperinsulation are concerned, the DC stress distribution determined by theaforementioned ρ characteristic is not the prominent factor of theirdesign, but these cables have rather been designed on the basis ofimpulse design as with AC cables, and more particularly on the basis ofthe electric stress which will develop when an impulse with oppositepolarity superposed onto DC is applied on the conductor.

For discussions on DC cables for large electric power transmission, notonly is impulse strength required, as in AC cables, but DCwithstand-voltage strength is required as well. In this case, evaluatingthe necessary thickness of the insulation would generally result in alarge thickness, detracting from ensuring the required transmissioncapacity. As a countermeasure, the conductor resistance may be reducedby increasing the conductor size, to cut down its current loss, therebyensuring the transmission capacity. But this approach would involveproductivity reduction, cable size increase, cost rise and other variousproblems.

DISCLOSURE OF THE INVENTION

Against this background, the present invention provides a DC OF cable inwhich the above described problems have been solved, Its firstcharacteristic feature lies in that its main insulation is composed of acomposite insulation tape (hereinafter abbreviated to PPLP) formed bylaminating a low dielectric loss plastic film and kraft paper, thatρ-grading is applied by combining therewith another or other PPLP(s)differing in plastic film ratio, as required, and that 1-10 sheets orlayers of kraft paper are wound on the aforementioned PPLP maininsulation to arrange its layer(s) on the inside and/or outside thereof.

Its second characteristic feature is that its main insulation iscomposed of PPLP, that ρ-grading is applied by combining therewithanother or other PPLP(s) differing in plastic film ratio, as required,and that 3-5 sheets or layers of carbon paper are wound on theaforementioned PPLP main insulation to arrange its layer(s) on theinside and/or outside thereof.

Further, its third characteristic feature lies in that its maininsulation is composed of PPLP, that ρ-grading is applied by combiningtherewith another or other PPLP(S) differing in plastic film ratio, asrequired, that 3-5 sheets or layers of carbon paper are wound at leastjust onto the conductor, and that 1-10 sheets or layers of kraft paperare wound on the PPLP insulation to arrange its layer(s) on the insideand/or outside thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an insulation configuration of a DCOF cable embodying this invention;

FIG. 2 is a cross-sectional view of an insulation configuration ofanother DC OF cable embodying this invention;

FIG. 3 is a cross-sectional view of an insulation configuration of stillanother DC OF cable embodying this invention; and

FIG. 4 is a DC characteristic comparison between PPLP and conventionalkraft paper.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a DC OF cable in which its maininsulation is composed of a composite tape formed by laminating a lowdielectric loss plastic film and kraft paper, and 1-10 sheets of kraftpaper are wound on the aforementioned composite tape main insulator toarrange its layer(s) on the inside and/or outside thereof.

The PPLP commonly applied to AC cables is a composite insulation formedby integrally laminating low dielectric loss polyolefinic film, forexample, polypropylene film (designated as PP) and kraft paper(s), isone that gives low dielectric loss as well as high dielectric strengthand that has superior impulse characteristic to kraft paper.

The DC characteristic of this PPLP is confirmed to be very excellent, asshown in FIG. 4. As a measure for improvement in the DC characteristicof kraft paper, increasing its impermeability is mentioned. PPLP has animpermeability such as is equivalent to infinity. Because as the PPratio in PPLP grows larger, its impulse and DC characteristics areenhanced, it becomes possible to form an optimal ρ-distribution(ρ-grading) through the optimization of the ρ-distribution and stress ineach part of the insulation by combining appropriate PPLP layer(s) whichare composed of PPLP with different PP ratios.

Application examples are as follows: The measure is not always one inview of conductor size and cable load (transmission current). The DCstress is high on the conductor side at normal temperature, but it ishigh on the sheath side at high temperatures. On this account, a PPLPhaving high dielectric strength, such as one with an approx. 60% PPratio, i.e. a ratio of the content of polypropylene film to the PPLP asa whole of 60%, is arranged on the conductor side of the main insulationand on the sheath side of the main insulation, where stress is high, anda PPLP with an approx. 40% PP ratio is arranged intermediatetherebetween. In the case where an evenly averaged stress is optimal,PPLP with an approx. 40% PP ratio is arranged on the conductor side andon the sheath side, and PPLP with an approx. 60% PP ratio is arrangedintermediate therebetween.

As hereinabove described,.where the DC stress distribution is concerned,stress is high on the conductor side at normal temperature but on thesheath side at high temperatures. Generally, as for the strength ofinsulation, the thinner the insulating tape, the higher itswithstand-voltage strength. Accordingly, by arranging thin insulatingtapes on the conductor side and the sheath side, it is possible torespond to the stress distribution peculiar to DC cables as mentionedabove mentioned and design a cable capable of withstanding AC surge.

Next, as for impulse characteristic of PPLP, as shown in Table 1, PPLPgives rather large polarity differences between (+) and (-). This can beconsidered to stem from the PP film contained in PPLP.

                  TABLE 1                                                         ______________________________________                                                                 DC Cellulose                                         Item           PPLP      paper                                                ______________________________________                                        Thickness (μm)                                                                            125       125                                                  Film ratio (%) approx. 50                                                                              --                                                   ε      2.8       4.0                                                  tanδ (%) 0.08      0.33                                                 ρ.sub.o (Ω · cm)                                                          4 × 10.sup.19                                                                     6 × 10.sup.18                                  α (1/°C.)                                                                       0.07      0.10                                                 β (mm/kV) 0.11       0.045                                               Breakdown strength (kV/mm)                                                    AC             53(80)     44                                                  Imp+           114(140)  115                                                  Imp-           156(209)                                                       DC+            149(211)  134                                                  DC-            136(232)                                                       ______________________________________                                    

As a result of assiduous research, we discovered a very useful fact.When a few sheets of kraft paper are arranged on the (+) polarity sideof PPLP, improvement will be effected in the direction of diminishingpolarity difference. Further, improvement was found to be achieved byarranging 3-5 sheets of carbon papers there with about 10³ -10⁶ Ω.cm.

It was found out as a result of evaluations pursued on a variety ofconfigurations that use of 3 or more sheets, or preferably 7-10 sheets,of approx. 100 μm kraft paper with a 1000 Gurley sec or higher ofimpermeability and 3-5 sheets of carbon paper of about 10³ -10⁶ Ω.cm isrecommended and that a configuration of conductor-carbon paper(s)-kraftpaper(s)-PPLP(s) kraft paper(s)-carbon paper(s)-metal sheath isgenerally most preferable for DC OF cables. However, use of still moresheets of carbon paper and kraft paper will result in thickerinsulation, raising problems in insurance of permissible current andeconomy. Therefore, a still larger number of sheets should be avoided.

Further, in connection with the countermeasure against (+) polarityimpulse problem, since the stress on the conductor side grows severerwith increasing conductor size (for example, to 600 mm² or larger) andinsulation thickness (for example, to 10 mm or larger), a configurationof conductor-carbon paper(s)-kraft paper(s)-PPLP(s)-metal sheath, thatis, the aforementioned configuration with kraft paper(s) and/or carbonpaper(s) omitted on the sheath side where less stress is involved, isalso practical.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 presents a cross-sectional view of a concrete example of theinsulation configuration of a DC OF cable of this invention. Referringto this figure, reference numeral 1 denotes a cable conductor having anoil passage in its interior; 2, a usual internal shielding layer formedby winding the conductor with about 1-2 sheets of carbon paper; 3, alayer of 1-10 sheets of kraft paper wound thereabout; and 4, a PPLPinsulation layer having, for example, a PPLP with an about 60% PP ratiowound on the cable conductor side 43 and the sheath side 41 and anotherPPLP with an about 40% PP ratio wound at an intermediate portion 42therebetween, thereby providing a ρ grading. Numeral 5 designates 1-10sheets of kraft paper wound on the outside of the PPLP insulation layer4, to form its layer thereon, and 6 stands for a usual externalshielding layer formed on the last mentioned layer by winding it withabout 1-2 sheets of carbon paper.

FIG. 2 is a cross-sectional view of another concrete example of theinsulation configuration of the DC OF cable of this invention. Referringto this figure, the same numbers represent the same parts. In thisconcrete example, in place of the usual internal shielding layer on theconductor 1, 3-5 sheets of carbon paper are wound thereabout to arrangea carbon paper winding layer 7; then a PPLP insulation layer 4, on whicha ρ-grading similar to that of FIG. 1 is applied, is provided thereon;and further, a usual external shielding layer 6 is provided on topthereof. In this instance, in place of the external shielding layer 6,3-5 sheets of carbon paper may be wound thereabout to arrange a carbonpaper winding layer.

FIG. 3 is a sectional view of still another insulation configuration ofthe DC OF cable of this invention. In this figure, the same numeralsidentical to those in FIGS. 1 and 2 represent the same parts.

In this concrete example, on a cable conductor 1, 3-5 sheets of carbonpaper are wound to form a carbon paper winding layer 7; 1-10 sheets ofkraft paper are wound thereabout to arrange a kraft paper winding layer3; and then a PPLP insulation layer 4 on which a ρ-grading is appliedjust as hereinabove described is provided thereon. On top of it, 1-10sheets of kraft paper are wound to arrange a kraft paper winding layer 5and further 3-5 sheets of carbon paper are wound thereabout to arrange acarbon paper winding layer 8. In this instance, in place of the carbonpaper winding layer 8, for example, 1-2 sheets of carbon paper may bewound therearound to provide an external shielding layer.

EFFECT OF THE INVENTION

When large capacity transmission cables adapted for use at ±500 kV and2800 A are designed using a DC OF cable of this invention as shown inFIG. 3 and a prior art DC OF cable configured with the use of kraftpaper, they compare as shown in Table 2. Table 2 indicates that in theformer, the cable outer diameter and weight can be reduced, conductorsize decreased, cable cost reduced, and moreover, the installation workexpense can be retrenched. Accordingly, its effects are quite enormous,when it is utilized as long distance submarine power cables for largeelectric power transmission.

                  TABLE 2                                                         ______________________________________                                                               Prior art                                              Item       Unit        paper    PPLP                                          ______________________________________                                        Size       mm.sup.2    3500     3000                                          Insulation mm           25      20                                            thickness                                                                     Outer      mm          182      166                                           Diameter                                                                      Weight     kg/m        101      86                                            Cost       %           100      98                                            ______________________________________                                    

What is claimed is:
 1. A DC OF cable, comprising:a cable conductorhaving an interior oil passage; a main insulation surrounding said cableconductor, said main insulation comprising a composite tape that iscomposed of a low dielectric loss plastic film laminated with kraftpaper; and a carbon winding layer comprising 3-5 sheets of carbon paperlocated at at least one of a first position inside said main insulationand outside of said cable conductor and a second position outside ofsaid main insulation.
 2. The DC OF cable of claim 1, wherein said lowdielectric loss plastic film comprises polypropylene film, and the ratioof said polypropylene film in said main insulation varies according tothe radial position of said composite tape relative to said cableconductor such that said main insulation is ρ-graded.
 3. The DC OF cableof claim 2, wherein the ratio of polypropylene film in said maininsulation is 60% at an inner radial position thereof adjacent to saidcable conductor, 60% at an outer radial position thereof and 40% at aradial position thereof between said inner and outer radial positions.4. The DC OF cable of claim 1, wherein said carbon winding layer islocated at said first position as an internal shielding layer, with said3-5 sheets of carbon paper being wound on said cable conductor.
 5. TheDC OF cable of claim 4, wherein a second said carbon winding layercomprising 3-5 sheets of carbon paper is provided at said secondposition, wound on said main insulation.
 6. The DC OF cable of claim 4,wherein an external shielding layer of about 1-2 sheets of carbon paperis wound on said main insulation.
 7. The DC OF of claim 4, wherein saidmain insulation is wound on said carbon winding layer.
 8. The DC OF ofclaim 1, wherein said carbon winding layer is provided at said secondposition, wound on said main insulation.
 9. A DC OF cable, comprising:acable conductor having an interior oil passage; a main insulationsurrounding said cable conductor, said main insulation comprising acomposite tape that is composed of a low dielectric loss plastic filmlaminated with kraft paper; a carbon winding layer comprising 3-5 sheetsof carbon paper arranged on said cable conductor between said maininsulation and said cable conductor; and a kraft paper winding layercomprising 1-10 sheets of kraft paper arranged at at least one of afirst position inside said main insulation and outside of said cableconductor and a second position outside of said main insulation.
 10. TheDC OF cable of claim 9, wherein said low dielectric loss plastic filmcomprises polypropylene film, and the ratio of said polypropylene filmin said main insulation varies according to the radial position of saidcomposite tape relative to said cable conductor such that said maininsulation is ρ-graded.
 11. The DC OF cable of claim 10, wherein theratio of polypropylene film in said main insulation is 60% at an innerradial position thereof adjacent to said cable conductor, 60% at anouter radial position thereof and 40% at a radial position thereofbetween said inner and outer radial positions.
 12. The DC OF cable ofclaim 9, wherein said 3-5 sheets of carbon paper are wound on said cableconductor and said 1-10 sheets of kraft paper are wound on said carbonpaper winding layer at said first position.
 13. The DC OF cable of claim12, wherein a second said kraft paper winding layer comprising 1-10sheets of kraft paper is arranged at said second position, with saidsheets of kraft paper of the second said kraft paper winding layer beingwound on said main insulation.
 14. The DC OF of claim 13, wherein asecond carbon paper winding layer comprising 3-5 sheets of carbon paperis wound on the second said kraft paper winding layer.
 15. The DC OF ofclaim 13, wherein an external shielding layer comprising 1-2 sheets ofcarbon paper is wound on the second said kraft paper winding layer.